This invention relates to thin film electroluminescent devices, and more particularly to thin film electroluminescent devices that utilize new insulator materials and which are made using new processes.
Thin film electroluminescent (TFEL) display devices are well-known. Typically, such devices consist of a substrate on which is deposited a first electrode layer. A first insulator layer, a layer of phosphor, and a second insulator layer are thereafter sequentially deposited on the first electrode layer. A second electrode layer is then deposited on the second insulator layer. A voltage applied across the first and second electrode layers causes the phosphor to luminesce.
TFEL display devices suffer from some common problems. First, the panels are subject to electrical breakdown. Arcing, or xe2x80x9cburn-outs,xe2x80x9d may occur between the electrode layers, thus creating areas of the device which do not luminesce. While some insulating materials fuse and limit the extent of the electrical breakdown, other insulating materials allow the electrical breakdown to propagate throughout the material, thus creating large areas of the device which do not luminesce. Further, if breakdown occurs across an electrode row or column, the entire row or column will fail to luminesce.
TFEL display devices also have several problems related to display characteristics. The panels themselves are not particularly bright. Increasing luminance of the panels typically requires increasing power. However, increasing the power can lead to electrical breakdown. In addition, the luminance of the TFEL display devices has been found to vary over time. This is particularly true in applications involving dimming and gray scale operation. The TFEL devices also exhibit latent images when the pixels or dots should otherwise be on or off. Further, the devices exhibit nonuniformity, with different portions of the devices exhibiting different luminance at the same electrical potential.
In general, two different process types, chemical vapor deposition and physical vapor deposition, have been used to deposit the insulating layers on the TFEL devices. Chemical vapor deposition includes such processes as atomic layer epitaxy, molecular beam epitaxy, thermal CVD and plasma enhanced CVD.
In physical vapor deposition, materials may be evaporated or sputtered to deposit thin films on the electroluminescent devices. Diode sputtering refers to two different sputtering methods: direct current (DC) sputtering and radio frequency (RF) sputtering. In DC sputtering, the target is connected to a negative potential with a positively charged anode present in the deposition chamber. The negatively charged target ejects electrons, which accelerate toward the anode. Along the way they collide with argon gas items, ionizing them. The positively ionized argon atoms then accelerate to the target, initiating the sputtering process.
In RF sputtering, the target is connected to the negative side of a radio frequency generator. The ionization of the gas takes place near the target surface without requiring a conductive target. Radio frequency sputtering is necessary to sputter non-conductive materials and is used also for conductors. Biasing is used with the radio frequency sputtering to achieve a cleaning effect at the film surface. Etching and cleaning are achieved by putting the film at a different field potential than the argon, causing the argon atoms to impinge directly on the film. This procedure is called sputter etch, reverse sputter, or iron milling.
Traditionally, the same manufacturing process, whether physical vapor deposition or chemical vapor deposition, has been used to deposit both of the insulating layers on the TFEL device. For example, in one prior art TFEL display device, first and second insulating layers of silicon oxynitride (SiON) are deposited using RF sputtering (referred to herein as a conventional SiON TFEL device). This TFEL device provides good fusing characteristics in the event of electrical breakdown and good phosphor efficiency. However, the SiON films have many pinholes and provide marginal step coverage over the electrodes. The relatively poor step coverage of the electrodes requires additional thickness for the first insulating layer, and in fact the first insulating layer is generally twice as thick as the second insulating layer. The net result is that while phosphor efficiency in this TFEL device is good, nevertheless these devices require higher threshold voltage and have lower brightness. In addition, while the displays exhibit reasonable display stability, the display characteristics nevertheless change over time.
In another TFEL display device, both the first and second insulating layers are aluminum titanium oxide (ATO), and both are deposited using atomic layer epitaxy. These devices do not exhibit fusing during electrical breakdown, and therefore must be treated with great care. Any significant point contact with the active area results in catastrophic burn-out. Thus, even though ATO has greater electrical strength than SiON, the lack of fusing means that very high dielectric strength margins must be used. Thick films must be used to achieve dielectric strength margins about three times the nominal breakdown field. Phosphor efficiency is poor, and the phosphor uniformity is marginal in dimming or gray scale applications. These devices also experience changes in display performance over time.
Mizukami et al. U.S. Pat. No. 4,188,565 disclose TFEL devices having several different insulating layers. The layers are deposited using diode sputtering. The disclosed ability to conduct reverse sputtering indicates that RF sputtering is used to deposit the layers. Mizukami discloses several devices using tantalum oxide (Ta2O5) as an insulating layer. In one device, both insulating layers are comprised of a layer of tantalum oxide and a layer of SION. In each insulating layer, the layer of tantalum oxide is adjacent to the electrode, and the layer of SiON is adjacent to the phosphor layer In another embodiment, the first insulating layer is SiON and the second insulating layer is tantalum oxide. In yet another embodiment, the first insulating layer is SiON, and the second insulating layer is Y2O3.
Suntola et al. U.S. Pat. No. 4,389,973 disclose a method using atomic layer epitaxy to make a TFEL display device. Suntola discloses two devices. In the first, the two insulating layers are tantalum oxide (Ta2O5). In the second, both insulating layers are Aluminum Oxide (Al2O3). In both devices, the films are deposited using the same ALE process.
European Patent 0 229 627 B1 discloses a TFEL display device in which both of the insulating layers are Ta2O5. Both of the Ta2O5 layers are deposited by sputtering. Alternatively, the patent discloses Barium Titanate (BaTiO3) as the first and second insulating layers.
Yet another prior art TFEL device used a thick film of lead titanate for the bottom insulating layer. The device had a very thin top insulating layer, and in some instances no insulating layer.
Nevertheless, none of the prior art devices have satisfactorily provided a display that combines efficient display performance and high brightness, fusing capabilities in the event of electrical breakdown, and stable display performance over time.
In addition, the processes used to create TFEL devices can be expensive and time-consuming. In order to minimize the display variations and reduce latent image problems, the TFEL devices are typically xe2x80x9cburned-inxe2x80x9d by subjecting the panels to extended use at high voltages. However, the burn-in time needed to stabilize the image can be quite long, and the amount of power used to burn-in the panels is limited by the ability of the panel to withstand electrical breakdown.
In addition, the manufacturing processes themselves can be expensive. In particular, ALE is a time-consuming and expensive process due to the slow rate of film formation.
What is therefore desired is a TFEL display device which provides high luminance or which can be used at low power, which provides greater reliability and resistance to electrical breakdown, which exhibits fusing in the event of electrical breakdown, which provides uniform and stable display performance over time, and which is cheaper and easier to manufacture.
The present invention addresses the problems of the prior art. In a first aspect of the invention, a thin film electroluminescent device comprises a bottom substrate and a first electrode layer on the bottom substrate. A first insulating layer is on the first electrode layer, and at least a portion of the first insulating layer includes a layer of aluminum titanium oxide. A phosphor layer is on the first insulating layer. A second insulating layer is deposited on the phosphor layer, and at least a portion of the second insulating layer includes a layer of a fusing dielectric material. A second electrode layer is on the second insulating layer.
In another aspect of the invention, a thin film electroluminescent device has a bottom substrate. A first electrode layer is deposited on the bottom substrate. A first insulating layer is deposited on the first electrode layer. At least a portion of the first insulating layer includes a layer of a refractory metal oxide deposited using DC reactive sputtering, the metal oxide selected from the group consisting of zirconia, hafnia, tantala and niobium oxide. A phosphor layer is deposited on the first insulating layer. A second insulating layer is deposited on the phosphor layer, at least a portion of the second insulating layer including a layer of a fusing dielectric material. A second electrode layer is deposited on the second insulating layer. In a preferred embodiment, the first insulating layer includes a barrier layer between the first electrode layer and the refractory metal oxide layer. In another preferred embodiment, the first insulating layer includes a barrier layer between the refractory metal oxide layer and the phosphor layer. In still another preferred embodiment, the second insulating layer includes a phosphor interface layer.
In another aspect of the invention, a thin film electroluminescent device has a bottom substrate and a first electrode layer deposited on the bottom substrate. A first insulating layer is deposited on the first electrode layer. A phosphor layer is deposited on the first insulating layer. A second insulating layer is deposited on the phosphor layer, at least a portion of the second insulating layer including a layer of a fusing dielectric material and including a phosphor interface layer. A second electrode layer is deposited on the second insulating layer.
Still another aspect of the invention provides a method for manufacturing a thin film electroluminescent device. A first electrode layer is deposited on a substrate. A first insulating layer is deposited on the first electrode layer using DC reactive sputtering. At least a portion of the first insulating layer is a layer of a refractory metal oxide, where the metal oxide is selected from the group consisting of zirconia, hafnia, tantala and niobium oxide. A phosphor layer is deposited on the first insulating layer. A second insulating layer is deposited on the phosphor layer, at least a portion of the second insulating layer including a fusing dielectric material. A second electrode layer is deposited on the second insulating layer. In a preferred method, a barrier layer is deposited between the first electrode layer and the refractory metal oxide layer. In another preferred method, a barrier layer is deposited between the refractory metal oxide layer and the phosphor layer. In another preferred method, the second insulating layer includes a phosphor interface layer.
The foregoing and other objectives, features and advantages of the invention will be more readily understood upon consideration of the following detailed description of the invention, taken in conjunction with the accompanying drawings.